There are numerous applications and industries that are based on rotating shafts to accomplish some form of work or energy conversion. Early examples of rotating shaft functionality include the watermills and windmills thousands of years ago to grind grains. Rotating shafts are still used on current windmills and hydroelectric plants, however they incorporate advanced technology and processing. While small rotating shafts are used in electronic equipment such as computer disk drives, media recorders/players, and household appliances, these shafts are generally of a smaller length and width such that the torque is relatively small. Larger rotating shafts experience larger torque and are deployed in applications including locomotives, airplanes, ships, and energy conversion to name just a few examples. The modern usage of equipment utilizing larger rotating shafts typically incorporates sensing and processing capabilities to achieve safe and efficient operation.
One of the ways of addressing the design and operation of equipment using rotating shafts is via measuring the stress or strain at the shaft surface that can be used to measure torque, bending and twisting due to externally applied forces. Conventional technologies employ a number of different systems of sensing or measuring the torque such as strain gauge systems, encoder/tooth systems, acoustic wave systems, elastic systems, magnetostrictive systems, and magnetoelastic systems. Each of these systems has certain characteristics and applications.
Strain gauges provide for local strain measurements of the shaft and typically require some form of coupling to the rotating shaft that can be via a physical connection (e.g.: slip rings) or telemetry. The gauges generally suffer from low stability, have limitations in the bandwidth and tend to have calibration and environmental correction requirements. The limited operating temperature range of strain gauges limits their use in a harsh environment.
The encoder/tooth-wheel pickup style of torque sensing usually has at least some partial attachment to the rotating shaft such as by a magnetic tooth-wheel. The tooth-wheel design tends to be costly and impractical for many implementations. Such a design is not practical for higher speed applications and although stable, lacks high resolution and can cause reliability issues in harsh environment
The acoustic wave systems utilize sensors such as surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices that use acoustic waves to detect strain-induced changes to the shaft via telemetry with transducers connected on the shaft. The application of acoustic wave technology to torque sensing is relatively new and the present systems are being used for smaller shafts that have high manufacturing tolerances.
The elastic torque systems measure the twisting of the shaft by using markers across a length of the shaft and measuring the angular displacement. This system has accuracy issues when applied to large diameter shafts, and there are practical implementation problems.
In a magnetoelastic system, stress induces an ‘easy axis’ of magnetization through the strain it produces in the material. This effect is typically used in circularly magnetizing a shaft region, and using a magnetic field sensor to pick up the resulting field. If there is no torsional stress, all of the magnetic flux is contained within the sensing region and there is no external field detected. If there is torsional stress, the magnetic domains are re-aligned and the external field is changed with a polarity and strength that corresponds to the direction and magnitude of torque on the shaft.
Magnetostrictive measurement methods make use of the phenomenon that material changes dimensions upon being magnetized. Magnetostrictive sensors are used with ferromagnetic shafts, such as industrial steel, as well as sections of ferromagnetic material applied to the shafts. With such materials the magnetostrictive effect is very small. Typical magnetostrictive coefficients Δ1/1 are in the order of 1 to 25×10−6. Making direct use of the magnetostrictive effect for measuring torque in ferromagnetic material requires complex sensor arrangements, difficult calibration procedures and typically results in limited accuracy.
One conventional magnetostrictive torque sensor design employs a primary coil in the center of the measuring head and measurement coils disposed about the periphery in a specific orientation. The sensor generates a constant high frequency magnetic field via the primary coil wherein the resulting field is measured by measurement coils that measure the magnetic flux. When the resulting magnetic field from all the measurement coils is equal to zero, there is no torque on the shaft and likewise, any resulting magnetic field that is non-zero indicates some torque is present.
This approach does not require any encoding or other modifications to the shaft and does have long-term stability. However, the accuracy is limited, the installation process is cumbersome and calibration tends to be difficult. Furthermore, there are generally tight tolerance requirements for keeping a small gap between the shaft and the sensor that is difficult to achieve with temperature varying environments. The accuracy of the measurements is acceptable in certain requirements such as monitoring dynamic torque components and torsional vibrations however subsequent signal processing is required to minimize shaft run-out issues.
Improving the accuracy of magnetostrictive measurement systems can be achieved in combining the magnetostrictive effect with a magnetic encoding of the shaft or the encoding section applied to the shaft. In such sensor designs the alignment of the magnetic domains in the ferromagnetic material imparts some change of the material dimensions along the magnetic axis. The inverse effect is the change of magnetization of a ferromagnetic material due to mechanical stress. The magnetic encoding essentially turns the shaft into a component of the sensing system. When a mechanical torque is applied to the shaft, a torque-dependent magnetic field is measurable close to the encoded region of the shaft.
A typical magnetostrictive torque sensor design employs total shaft encoding and the magnetization occurs by current flowing in the axial direction of the shaft. For illustrative purposes, this conventionally uses current pulses to create a “magnetized ring” about the shaft. The encoding is circumferentially uniform as the magnetic encoding requires the entire cross-section to be magnetized and therefore becomes difficult and costly for larger diameter shafts. In addition, there are limitations to this approach with respect to variations of currents due to inhomogeneity of electrical and magnetic properties of the shaft. As torque is applied, a torque dependent magnetic field can be measured externally, such as by fluxgate sensors.
There are known implementations for magnetostrictive encoding which can be used, for example, in the automotive industry. The shaft is typically axially encoded along axial sections of the shaft establishing magnetically encoded regions. Magnetic field sensors are deployed externally to the magnetically encoded region, and the measured responses are subsequently processed for the torque. While there are a number of various other embodiments, the descriptions herein illustrate the basic operation.
Referring to FIG. 1a, a shaft 5 is composed of a ferromagnetic material. In order to encode the shaft 5, electrodes 10, 15 are disposed along the circumference of the shaft such that an encoded region 20 can be formed. In this example, the electrodes consist of a pair of outer rings 10, 15 and are spaced apart to provide satisfactory uniform magnetic flux density during encoding that depends upon several factors such as the shaft diameter.
The encoding process typically involves sending a current pulse 25 to a first ring 15 that establishes a current flow 30 along the longitudinal axis of the shaft 5 that is discharged at a corresponding second ring 10 with an output return current signal 35. The current 30 flowing in the encoded region 20 through the shaft 5 induces a magnetic flow. Various embodiments implement this basic concept including using multiple electrodes and with various encoding techniques.
In operation, a sensor is used to measure an output magnetic field signal that reflects the torque applied to the shaft. With no stress applied, there is no relevant magnetic field detected however as torque is applied to the shaft, the change in magnetic field emerging from the encoded region is measured by the sensor. The sensor is typically coupled to some processing electronics.
Referring to FIGS. 1a-1b, the current 30 flowing in the encoded region 20 through the shaft 5 creates a magnetic flow 40 that is a magnetic field at a center of the shaft 5.
One embodiment for illustrative purposes uses a single current pulse for creating a “magnetized ring” within the shaft such that there is an associated discharge curve reflecting the characteristics of the shaft at a particular instance of time. Alternatively, consecutive pulses with different polarities and different time constants can be utilized such that two magnetized rings can be encoded. There can be multiple encoding electrodes in addition to the encoding pulse.
This conventional system employs uniform encoding of the surface of the shaft and the magnetization occurs by current flowing in the axial direction of the shaft. The magnetic encoding is circumferentially uniform and requires the entire cross-section of the shaft to be magnetized. Such encoding makes it difficult to achieve a uniform current distribution in circumferential direction, especially with large diameter shafts.
With no stress applied, there is essentially no magnetic field detected however as torque is applied to the shaft the magnetic field emitted by the encoded region is measured by the sensor 45. In operation, as torque is applied, the magnetic field is measured externally, such as by sensor coils 45. The sensor 45 is typically coupled to some processing electronics (not shown) and is typically used to measure the output magnetic field signal that reflects the torque applied to the shaft 5. An example of a sensor 45 for torque sensing is a fluxgate sensor that is installed close to the shaft surface.
The conventional magnetic shaft encoding described herein generally applies to small diameter shafts with an encoding that is based on a uniform (constant) flux density in the circumferential direction. It is not practical for larger shafts as the encoding currents increase with the shaft diameter and large amperage would be required in order to get sufficient flux densities in the large diameter shafts.
In order to attempt to alleviate these large diameter shaft concerns, one conventional method uses multi-channel electrical connections as shown in FIG. 1c. In this example, a pair of rings 50, 55 are disposed proximate the shaft 5 with multiple electrical connections 60 electrically coupling to the shaft 5 such that the input current signals 65 travel along a magnetic encoding section length 80 of the shaft 5 with the return output signal 70 such that the encoding defines a magnetized region 75 in the shaft.
This complex encoding arrangement requires the spacing between the individual circumferentially placed current entry points be small in relation to the shaft diameter. Otherwise a sufficiently uniform magnetization in a circumferential direction is not achievable. Larger spacing requires the section length 80 be larger which causes implementation problems in many applications. In addition, the individual currents applied to the electrical connections must be controlled to all have the same amplitudes which becomes costly for larger diameter shafts.
Referring to FIG. 1d and FIG. 1e, the conventional magnetoelastic sensing of torque 90 is illustrated in which there are polarized rings 92, 93 that are coupled about the shaft 94 such that the rings 92, 93 magnetically divide opposing polarization regions. In this example, a domain wall 98 separates the polarized rings 92, 93. A magnetic field sensor 95 is located proximate the rings 92, 93 and senses the magnetic flux density 96. The results from the sensor 95 are processed such that the stresses in the rings 92, 93 correspond to torque imparted upon the shaft 94. Typically the results are transmitted to a computing device 99 such as a computer for the post processing. Once again, applying this type of sensing it typically inefficient for large diameter shafts.
The conventional sensing systems such as shown in FIGS. 1a-e generally operate on the principles referred to as the inverse Joule effect, the Matteucci effect, the Wertheim effect, the Villari effect and the inverse Wiedemann effect such as detailed in “A Study of the Inverse Wiedemann Effect on Circular Remanence” by I. J. Garshelis and J. Ivan, IEEE Transactions on Magnetics, Vol. 10, No. 2, June 1974. These effects are related to magnetostriction that explains changes in volume of magnetized material when torque is applied, thus explaining the connection between mechanics and magnetics.
Various processes and systems have been used to provide accurate and reliable measuring capabilities for rotating shaft, however continued improvements are needed especially with respect to larger diameter shafts and enhancements in operational efficiency.